1. Field of the Invention
The present invention relates to a process for upgrading a liquid petroleum or chemical stream wherein said stream flows countercurrent to the flow of a treat gas, such as a hydrogen-containing gas, in at least one reaction zone. The temperature of at least a portion of the liquid stream in the reactor is used to control the flooding characteristics of the reactor.
2. Background of the Invention
There is a continuing need in the petroleum refining and chemical industries for improved catalysts and process technology. One such process technology, hydroprocessing, has been subjected to increasing demands for improved heteroatom removal, aromatic saturation, and boiling point reduction. More active catalysts and improved reaction vessel designs are needed to meet this demand. Countercurrent reaction vessels have the potential of helping to meet these demands because they offer certain advantages over co-current flow reactors. Countercurrent hydroprocessing is well known, but of very limited commercial use. A countercurrent process is disclosed in U.S. Pat. No. 3,147,210 that teaches a two-stage process for the hydroprocessing-hydrogenation of high boiling aromatic hydrocarbons. The feedstock is first subjected to catalytic hydroprocessing, preferably in co-current flow with hydrogen. It is then subjected to hydrogenation over a sulfur-sensitive noble metal hydrogenation catalyst countercurrent to the flow of a hydrogen-rich gas. U.S. Pat. Nos. 3,767,562 and 3,775,291 disclose a similar process for producing jet fuels, except the jet fuel is first hydrodesulfurized prior to two-stage hydrogenation. U.S. Pat. No. 5,183,556 also discloses a two-stage concurrent-countercurrent process for hydrofining-hydrogenating aromatics in a diesel fuel stream.
While the concept of countercurrent hydroprocessing has been known for some time, countercurrent flow reaction vessels are typically not used in the petroleum industry, primarily because conventional countercurrent reaction vessels are susceptible to catalyst bed flooding. That is, the relatively high velocity of the upflowing treat gas prevents the downward flow of the liquid. The liquid thus cannot pass through the catalyst bed. While flooding is undesirable, catalyst contacting by the reactant liquid improves as the bed approaches a flooded condition. However, operating close to the point of incipient flooding leaves the process vulnerable to fluctuations in pressure or temperature or in liquid or gas flow rates. This could result in a disturbance large enough to initiate flooding, and process unit shutdown in order to recover stable operation. Such disruptions are highly undesirable in a continuous commercial operation.
In a countercurrent flow reactor temperature gradients are produced: axial, radial, and localized hot spots. Axial temperature gradients cause refluxing which decreases the kinetic efficiency of the reactor and can make the reactor hydraulically inoperable. All of the temperature gradients contribute to the potential for reactor runaway, decreased reaction selectivity, and less than optimum kinetic/thermodynamic performance. The potential for runaway is particularly important as it restricts the usage of catalysts systems such as those that promote hydrocracking. Conventional temperature controlxe2x80x94quench (gas or liquid) and inter bed heat exchangexe2x80x94does not fully solve these problems
Therefore, there still exists a need for improved countercurrent process designs to improve on the flooding characteristics, as well as other aspects of the process.
In accordance with the present invention there is provided a process for hydroprocessing a hydrocarbonaceous feedstream, which process comprises:
(a) introducing said feedstream into a reaction vessel upstream from at least one reaction zone and passing said feedstream through one or more reaction zones operated at hydroprocessing, wherein each reaction zone contains a bed of hydroprocessing catalyst;
(b) introducing a hydrogen-containing treat gas at the bottom of said reaction vessel and passing it upward through each reaction zone countercurrent to the flow of liquid feedstream, thereby reacting with said feedstream in the presence of said hydroprocessing catalysts and resulting in a liquid phase product stream and a vapor phase product stream;
(c) passing the liquid phase product out of the bottom of said reaction vessels;
(d) removing the vapor phase product stream overhead of said reaction zones; and
(e) controlling the temperature of the reaction vessel with one or more heat exchange devices either internal or external of said reaction vessel.
In a preferred embodiment the temperature of the reaction vessel is controlled with one or more heat exchange devices internal of said reaction vessel, which one or more heat exchange devices are selected from:
i) reaction zones that are vertically situated tubes, said tubes containing said hydroprocessing catalyst through which said feedstrean flows countercurrent to the treat gas and wherein a heat exchange media is circulated on the outer surfaces of said tubes;
ii) vertically situated tubes containing heat exchange media with said hydroprocessing catalyst being external to said tubes; and
iii) one or more reaction zones being divided into vertical sections with catalyst being situated in alternating sections and heat exchange media in the remaining sections.
In another preferred embodiment of the present invention the reaction vessel is controlled so that the entire reaction vessel is operated under substantially isothermal conditions.
In still another preferred embodiment at least a portion of the heat exchange media used in the heat exchange device is all or a portion of the hydrocarbonaceous feedstream.
In yet another preferred embodiment the temperature profile of the reaction vessel is controlled so that the temperature decreases as the hydrocarbon liquid proceeds down the reaction vessel.